The c-myc proto-oncogene plays a central role in normal cell proliferation and programmed cell death (Y. Shi, J. Glynn, L. Guilbert, T. Cotter, R. Bissonnette, and D. Green, "Role for c-myc in activation-induced apoptotic cell death in T cell hybridomas," Science, 257:212-214 (1992)) and its deregulation contributes to the formation of a variety of tumors. (J. M. Bishop, Annu. Rev. Biochem. 52,301-354 (1983); M. D. Cole, "The myc oncogene: its role in transformation and differentiation." Annu. Rev. Genet. 20, 361-384 (1986); S. Cory, Adv. Cancer Res. 47, 189-234 (1986)).
Down regulation of the c-myc proto-oncogene occurs in the human promonomyelocytic leukemia cell line HL60 and human monoblastic line, U937, upon induction of differentiation. (C. Dony, M. Kessel, and P. Gruss, Nature. 317, 636-639 (1985); L. E. Grosso, and H. C. Pitot, Cancer Res. 45, 847-850 (1985); T. Watanabe, E. Sariban, T. Mitchell, and D. Kufe, Biochem. Biophys. Res. Commun. 126, 999-1005 (1985); D. L. Bentley, and M. Groudine, Nature, 321, 702-706 (1986); D. Eick and G. W. Bornkamm, Nucleic Acids Res. 14, 8331-8346 (1986); T. Endo, and B. Nadal-Ginard, Mol. Cell. Biol. 6, 1412-1421 (1986)). This suppression of c-myc expression occurs by two mechanisms; within three hours there is a block to elongation which can be reversed by removal of the differentiation agent. Subsequently, transcriptional initiation ceases, coinciding with irreversible commitment to the differentiation pathway. (U. Siebenlist, P. Bressler, and K. Kelly, Mol. Cel. Biol. 8, 867-874 (1988)).
A Far Upstream Element ("FUSE") which is required for maximal transcription of c-myc, binds a factor (DROME or FUSE binding protein ("FBP")) which is present in extracts of undifferentiated cells, but disappears upon differentiation. (M. I. Avigan, B. Strober, and D. Levens, "A Far Upstream Element Stimulates c-myc Expression In Undifferentiated Leukemia Cells." J. Biol. Chem. 265, 18538-18545 (1990)). The disappearance of this binding activity occurs 24 hours after addition of the differentiation agent coinciding with the loss of initiation of c-myc transcription. The FUSE site differs from other described positive regulatory elements for myc in a number of ways. Despite its placement a long distance from the transcription start site (-1500 bp relative to the myc P1 promoter), the FUSE element will not act as a traditional enhancer; multiple copies inserted upstream of a heterologous promoter do not stimulate transcription in transection experiments. However, when the FUSE site is present with additional c-myc regulatory sequences, specific stimulation of the c-myc promoter is observed, indicating that FUSE works in concert with other myc elements. These features suggest that the factor binding to this site may act to stimulate transcription by an unusual mechanism.